The present invention relates to a communications device which is connected to a power line supplied with a predetermined a.c. voltage and performs data communication over the power line, as well as to a communications method.
When wired data communication is performed in home, in office, in a factory, or the like, by use of a terminal, such as a computer, wires to be used as transmission lines, such as cables and connectors, usually must be laid at required locations. Hence, various tasks must be carried out before operation of communications facilities is commenced.
Incidentally, in most cases, commercial power; e.g., an a.c. voltage (for example, 120V-60 Hz in the United States, 100V-50/60 Hz in Japan) is used in the home, the office, a factory, or the like. Accordingly, a power line (a lighting circuit) used for supplying the power has already been laid at all locations in the house, the office, the factory, and the like. Consequently, so long as the power line can be utilized for data communication, a necessity for newly laying a special wire for communication purpose is obviated. Specifically, a communications channel can be ensured by means of inserting a communications device into the power outlet.
A technique described in, e.g., Patent Document 1, has been known as a technique for utilizing such a power line for communication. Under present circumstances, conditions for utilizing power line communication (e.g., a frequency band) are under consideration in various countries, including Japan.
[Patent Document 1] JP-A-2000-165304
As things stand, specifications have not yet been defined in connection with a technique for utilizing a power line for communication such as that mentioned previously. Specifications of a communications standard, such as a protocol, a modulation system, and a frequency band, which are used for actual communication, vary according to a manufacturer which develops the technique.
When consideration is given to the environment where such a communications technique is actually used, there is a high probability that a plurality of types of communications standards mixedly exist in the same location. For instance, on the assumption that users (users of communications devices) reside in multiple housing, such as an apartment house or a condominium, the users residing in the same collective housing do not always use communications devices (e.g., modems) of the same manufacturer. Accordingly, there may arise a case where a plurality of types of communications devices uniquely manufactured by a plurality of manufacturers are simultaneously connected to the common power line.
When a plurality of types of communications devices, which differ from each other in terms of communications standards, such as a protocol and a modulation system, are connected to the same power line as mentioned above, a communications device cannot demodulate signals transmitted from communications devices of different systems and recognizes the signals as mere noise. Consequently, despite the fact that the plurality of types of communications devices use the same frequency band, mere presence of the other communications devices cannot be ascertained. For this reason, signals transmitted by communications devices of a plurality of types collide against each other, thereby rendering communication impossible. Specifically, difficulty is countered in coexistence of different types of communications devices on the common power line.